1. Field of the Invention
The present invention relates to a (co)polycarbonate having a low terminal hydroxyl group concentration which is obtained by adding an ester compound in the melt-polycondensation of a dihydroxy compound with a carbonic diester in the presence of one or more catalysts selected from among nitrogen-containing basic compounds, alkali metal borates and alkaline earth metal borates to thereby block the terminal hydroxyl group (hydroxyl residue) of the thus formed (co)polycarbonate with an ester group, and a process for producing the same.
The present invention also relates to a (co)polycarbonate composition comprising a borate and a linear, high-molecular weight (co)polycarbonate which is excellent in heat resistance, hydrolysis resistance, hue and impact resistance and which is obtained via polycondensation of a dihydroxy compound with a carbonic diester in the presence of a specific transesterification catalyst, and a process for producing the same.
The present invention further relates to a (co)polycarbonate composition comprising boric acid and/or ammonium hydrogenphosphite and a (co)polycarbonate which is obtained via polycondensation of a dihydroxy compound with a carbonic diester in the presence of a transesterification catalyst, and a process for producing the same.
2. Description of the Related Art
A "high-molecular-weight" polycarbonate is a general-purpose engineering thermoplastic which is useful in various fields, particularly as injection molding material or sheet material substituting for window panes. It is said that the polycarbonate usually has excellent thermal resistance, transparency and impact resistance.
Generally known processes for producing a polycarbonate include, for example, the phosgene process wherein a dihydroxy compound is reacted with phosgene by interfacial polycondensation and the transesterification process wherein a dihydroxy compound is reacted with a carbonic diester in a molten state.
The phosgene process, i.e., the interfacial polycondensation process, is generally effective in preparing a polycarbonate, but has disadvantages that the use of toxic phosgene is necessitated and that the formed polycarbonate is contaminated with residual chloride ion.
In order to overcome these disadvantages, Japanese Patent Publication-A No. 182336/1988 discloses a process for the preparation of a polycarbonate which comprises using liquid trichloromethyl chloroformate, which is a dimer of phosgene, instead of the toxic phosgene and polycondensing it with a special dihydric phenol by the interfacial process.
However, this patent document does not give any specific information about the special dihydric phenol with the exception of 9,9-bis(4-hydroxyphenyl)fluorenes. Further, although Angew. Chem. 99, 922(1987) describes that a polycarbonate is prepared from 2,2-bis(4-hydroxyphenyl)propane by using triphosgene instead of the toxic phosgene, a reaction mechanism wherein phosgene is generated is also described therein.
A representative transesterification process comprises reacting a dihydric phenol with a carbonic diester in the presence of a transesterification catalyst under heating under reduced pressure while distilling off a phenol formed to prepare a prepolymer and then reacting the prepolymer under heating finally 290.degree. C. or above in a high vacuum while distilling off a phenol formed to obtain a polycarbonate having a high molecular weight (see U.S. Pat. No. 4,345,062).
It is known that in the transesterification process, a prepolymer is prepared in an ordinary tank reactor having stirring blades in the initial stage of the reaction and then the polycondensation reaction is conducted in, for example, a vented horizontal extruder in order to efficiently conduct the reaction to thereby obtain a polycarbonate having a high molecular weight.
However, the transesterification process has the problem that a polycarbonate having a high molecular weight has such an extremely high melt viscosity, unlike other engineering plastics, that a temperature as high as 280.degree. C. or above is necessitated for the reaction and so is a high vacuum (1 to 10.sup.-2 Torr) for distilling off the monohydroxy compound having a high boiling point formed, which makes the industrialization of the process difficult from the viewpoint of the equipment.
As examples of the polymerization catalysts to be used in the production of polycarbonates by the transesterification process, hydroxides, hydrides, oxides, alcholates, carbonates and acetates of alkali metals and alkaline earth metals are commonly cited. However, there is a problem that these basic catalysts remain in the final products and thus seriously deteriorate the heat resistance, hydrolysis resistance, residence stability in the molding machine, weatherability and hue of the polycarbonates.
One method for solving these problems comprises adding a third component to the reaction mixture to thereby weaken the effectiveness of the basic catalysts. For example, U.S. Pat. No. Des. 1,031,512 (published on Jun. 4, 1958) has disclosed that the above-mentioned problems can be avoided by adding a substance, which is capable of binding to a base, to a molten resin at around the final point of the transesterification to thereby neutralize the basic catalyst. Further, Japanese Patent Publication-A No. 175368/1992 has disclosed a method of adding an acidic compound to a reaction product. However, these methods suffer from another problem that a small amount of an additive can be hardly blended homogeneously with a resin of a high melt viscosity within a short period of time.
Another method for solving the above-mentioned problems comprises altering the type of the catalysts per se. For example, Japanese Patent Publication-B No. 20504/1971 has disclosed a method of adding tetrafluoroborate or hydroxyfluoroborate as the catalyst. However, these catalysts contain halogen atoms, which causes a fear of, for example, the corrosion of devices. Further, Japanese Patent Publication-A No. 124934/1990 has disclosed that the above-mentioned decomposition, i.e., the decomposition of the polycarbonate by heat, hydrolysis or the like, can be prevented by using a nitrogen-containing basic compound together with an alkali metal (or alkaline earth metal) compound and boric acid (or an ester thereof). However, three compounds should be used as the polymerization catalysts in this case, which makes this method troublesome. Furthermore, Japanese Patent Publication-A No. 51719/1985 has disclosed a process for producing a polycarbonate having a relatively light color with the use of a catalyst system comprising a nitrogen-containing basic compound and a boric compound. However, this catalyst system exhibits low catalytic activity.
Accordingly, it has been urgently required to establish convenient processes for producing a polycarbonate having a low terminal hydroxyl group concentration and a linear, high-molecular weight polycarbonate which is excellent in heat resistance, hydrolysis resistance, hue and impact resistance, via polycondensation of a dihydroxy compound with a carbonic diester in the presence of a transesterification catalyst.